(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) device having test contact pads for testing the LCD panel, and more particularly, to the structure of a LCD device for testing the LCD panel without damaging electrodes of the LCD device.
(b) Description of a Related Art
Methods for mounting a drive semiconductor chip for driving a LCD panel in a current LCD device include a TAB (tape automated bonding) technique and a COG (chip-on-glass) technique. The TAB technique is such that the drive semiconductor chip is mounted on a TAB tape which is then connected to electrode pads formed on the LCD panel. The COG technique is such that the drive semiconductor chip is directly connected to the electrode pads of the LCD panel. The latter has advantages of smaller dimensions and lower costs compared to the former.
In a LCD panel using the COG technique, since the electrode pads must be formed corresponding to the arrangement of the output electrodes of the drive semiconductor chip, design for a smaller pitch and smaller dimensions of the electrode pads are developed. In general, the pitch of output electrodes of the drive semiconductor chip is tens of micrometers (μm). For example, for a drive semiconductor chip having a side length of 20 mm and 400 in number of output electrodes disposed on the side, the pitch of the output electrodes is as small as 50 μm, as obtained by a simple arithmetic calculation.
Thus, in a test for a COG-based LCD panel before mounting thereon a drive semiconductor chip, a test probe is used which has input/output probe electrodes arranged at a pitch of tens of micrometers. However, such a test probe is difficult to manufacture, and a short-circuit failure is often caused in the conventional technique by the small pitch, involving a failure in the test operation such as a short-circuit failure.
In addition, since a high accuracy is required during the test operation in the alignment between the probe electrodes and the electrode pads (contact pads) having a small pitch and small dimensions, it takes a long time to align the probe electrodes with respect to the contact pads. Thus, there is a problem low throughput in the test operation.
The mechanical contact between the probe electrodes and the contact pads in the LCD panel may damage the contact pads in the LCD panel by abrasion of surface or attachment of impurities thereon. This may cause an electric malfunction in connection between the electrode pads and the output electrodes of the drive semiconductor chip to be mounted later.
In the test operation, an electrical-chemical reaction may arise to cause a corrosion or melting in the contact pads if a voltage of several volts or more is applied to the test probe in a moisture ambient. This problem is highly probable especially in the case of an active matrix LCD panel, because a high voltage around 10 volts is applied to the scan lines in the LCD panel.
Patent Publication JP-A-6-110072 describes a technique for solving the above problems in testing a LCD panel having the COG structure, wherein openings for the contact pads in the LCD panel have a larger length compared to the space between the contact pads in the LCD panel.
More specifically, with reference to FIG. 1 showing a partial front view of the LCD panel described in the publication, the LCD panel has a pair of glass substrates 11 and 21 for defining a plurality of pixel areas not shown, and a LC layer sandwiched therebetween. The counter substrate 21 has a window 25 for mounting the drive semiconductor chip 16 on the TFT substrate 11. The opening 4 is formed for exposing each internal lead 12 which connects the scan line or signal line formed on the TFT substrate 11 in the display area thereof and the electrode pads 13, whereby the probe electrode is contacted with the internal lead 12 through the opening 4 during the test operation. The opening 4 has a width which is somewhat smaller than the width of the internal lead 12, and a length which is larger than the space between adjacent internal leads 12.
A probe electrode is contacted with the internal lead 12 at the area within each opening 4 shown by hatching, wherein the probe contact areas are arranged in a staggered arrangement with respect to the centers of the openings 4. The staggered arrangement of the probe contact areas allows conventional probe electrodes to contact portions of the internal leads (or contact pads) 12 arranged at a small pitch without undesired electrical contact between the probe electrodes.
In addition, the location of the probe contact area which is apart from the electrode pad 13 affords the advantage of avoiding a short-circuit failure even in the case of abrasion of surfaces or attachment of impurities on the internal leads 12 by the contact of the probe electrodes.
The proposed structure of the LCD device, however, has yet a problem of disconnection or open-circuit failure of the internal leads 12 occurring at the probe contact area due to the abrasion or corrosion caused by the contact between the probe electrode and the internal lead 12. The disconnection of the internal lead 12 causes a malfunction of display image on the LCD panel due to the absence of signal transmission to the scan line or signal line.